Anyway, Cap 1 was about World War II, Cap 2 is about a government cover-up. Cap 3 should be about Captain America fighting against some sort of terror plot. I feel that Sin, Viper or Zemo could lead a reformed Hydra. That or it should be about the Serpent Society being hired to kill Captain America after he decides to poke his nose around Roxxon's dirty laundry.

Alternately, I could see Doctor Faustus being the head of a European far-right populist party who has connections to terrorism but tries to hide it and thus finds himself at odds with Captain America.

Regardless, having Cutthroat as a mook and Diamondback be a defector who joins SHIELD would be a welcome addition if they go with any of the above. If for whatever reason Marve hates Diamondback, then they can at least bring in Flag Smasher as a lone wolf terrorist.